The present disclosure relates generally to fuel and oil systems for aircraft systems and more particularly to bypass valves for heat exchangers.
Gas turbine engines operate during varied environmental conditions, including at temperatures below the freezing point of water. Additionally, it is possible for fuel to absorb water under various conditions. Thus, under certain conditions it is possible for ice to form within the fuel system. For example, intercontinental flights at high altitudes, such as from Beijing to London, frequently produce conditions for icing. Ice crystals may also form under certain conditions before the engine is operating, such as when in hangars or on airstrips. The ice crystals can plug fuel lines and orifices in the fuel system, which may degrade performance of the gas turbine engine or even cause an engine stall. As such, gas turbine engines are equipped with systems for eliminating or removing ice particles from fuel lines. For example, heat exchangers are often provided just before the fuel pump to eliminate any ice crystals. Heat exchangers are desirable because the ice is removed from the system altogether and does not require periodic clearing or cleaning.
Typical ice removal systems comprise a heat exchanger that imparts heat to the fuel from engine oil used to cool various components of the engine. However, such systems require time for the engine oil to heat up, thereby delaying the melting of any ice crystals. Furthermore, at high altitude conditions the heat exchanger may not be able to extract adequate heat from the heat source, such as the electric generator oil or engine oil, to melt the ice. Thus, in the event the heat exchanger becomes clogged with ice to the point where free flow of fuel is inhibited, a bypass valve opens causing the fuel to circumvent the heat exchanger. The bypass flow of fuel keeps the engine running until such time the problem can be rectified by other means or the aircraft can be landed. However, the bypass valve itself produces a potential bottleneck that can become clogged with ice. There is, therefore, a need for improved systems for preventing ice blockage within fuel systems.